1. Field of the Invention
The present invention relates to a method of producing coated slabs of metal, particularly strips of steel, in which a metal slab is guided through the bottom of a vessel filled with molten metal having the same or different composition as the metal slab, wherein the dwell time of the metal slab is selected in dependence on the melting bath level, the casting speed, the metal slab thickness and the preheating temperature of the metal slab in such a way that the molten metal deposited on the metal slab has a desired thickness of a multiple of the initial thickness of the metal slab, and wherein the metal slab with the layer crystallized onto the metal slab is subjected to a smoothing pass after emerging from the melting bath. The present invention also relates to an apparatus for carrying out the method.
2. Description of the Related Art
The method and apparatus of the type described above are generally used for producing coated metal slabs which are thinly coated with stainless steels, preferably strips of a steel of one steel quality or different steel qualities, for example, single-component materials or composite materials, particularly also composite materials of carbon steel.
DE 195 09 691 C1 discloses an inversion casting vessel and a method of producing thin metal slabs, particularly of steel, in which a metal strip is guided through the bottom of a vessel filled with melt and is pulled off after metal has crystallized onto the strip. The metal strip is conveyed guided by guide rolls through a duct to the melt in the container. After a layer of molten metal has crystallized onto the strip, the strip is conveyed above the vessel through smoothing rolls in which the strip with the layer crystallized thereon is smoothened to dimensions which are close to the final dimensions.
DE 195 09 681 C1 discloses another inversion casting device and a method for continuously producing strip-shaped sheet metal, particularly of steel, in which a core strip is guided through a melting bath of a metal in order to achieve a certain form of crystals and molten metal which deposit on the surface of the core strip. After the core strip has left the melting bath, the crystallized coating is advantageously immediately smoothened by means of a pair of smoothing rolls which are arranged above the melting bath.
However, the inversion casting devices are discussed in the two references mentioned above primarily with respect to the seal of the vessel relative to the entering strip in such a way that the melting bath is intensively cooled in the area of the slot like entry opening for the core strip in such a way that a temperature drop occurs in the meniscus wherein the two-phase area of melt/crystal has such a high viscosity that the meniscus assumes the function of a seal which renews itself. In view of this background, the two references mentioned above do not provide any indication concerning an optimum manner of carrying out the method and optimization of the surface of the produced strip when being treated by the pair of smoothing rolls.
DE 43 19 569 C1 discloses a method of producing strip material of metal and an apparatus for carrying out the method in which a sheet thickness tolerance of at most 2% can be maintained. For this purpose, the semifinished product having a width/thickness ratio of above 60 is subjected to a smoothing pass after leaving the melting bath. When the smoothing pass is carried out, the steel strip has a "pasty" surface (2 phases: melt and crystal) in accordance with the example and the formula T=T,sol+ax (T,li-T,sol), with 0.5 being selected for a, wherein the deposited layer has an average temperature of T=1497.degree. C.+0.5.times.(1507.degree. C.-1497.degree. C.)=1502.degree. C.). This condition means that the steel strip is still "pasty" at its surface when entering the pair of smoothing rolls, i.e., the steel strip is still in the two-phase area, i.e., liquid/solid, and, thus, does not have a purely solid phase.
This method condition of a crystallized layer with a "pasty surface and pasty core" has the disadvantage that the layer adhering to the core strip is already solidified to a relatively significant extent, on the one hand, while still having in its outer zone sufficient portions of liquid phase when entering the pair of smoothing rolls, on the other hand, so that the strip is significantly undercooled when travelling though the pair of smoothing rolls and, thus, there is the tendency of the formation of cracks in longitudinal direction as well as in transverse direction of the strip. This danger occurs increasingly with increasing casting and rolling speeds.